This paper presents a modelling method - GI-SIM (GRAI/IDEF-Simulation) - developed to meet the requirements of manufacturing system analysis and design. The GI-SIM method combines three important concepts of modelling methods (conceptual, functional and simulation) in an integrated structure. It enables a system analyst and designer to build system models from the basic functional specifications to the sub-activity dynamic simulation level. This integrated modelling method presents the global view of the manufacturing system in a single level grid and uses IDEF0 to model every activity centre. Then lowest levels of activity centre, described by the IDEF0 sub-activity models, can be translated into a simulation modelling tool to measure and evaluate system performance. A computer-based support tool has been developed to assist model building and connect model objects and elements. To demonstrate and validate the modelling method and computerized tools developed, a case study has been carried out in an electric motor manufacturing company.